Planck length

Planck length is the smallest possible size of anything in the universe. It's about 100 million billion billion times smaller than a millimeter, which is about the size of a small spec of dust. It is so small that it would take a million million million of them side-by-side to be as big as the Earth! So, it's really, really small. We can't see these tiny lengths, but they are an important part of understanding the universe.